Re: Using the --snaptime option

2014-01-24 Thread Albert Peschar
 That will effectively disable the recognize when files haven't changed
 functionality, which will force Tarsnap to re-read files which it might
 otherwise have not bothered to re-read.

That's the hint I needed to understand how it works.

Daniel, if I am not mistaken, the idea is to flag those files that were
modified around the time of snapshot creation as modified. Normally
files with the same modification time, size and path are assumed not to
have changed. And as Colin explained earlier, this can lead to a race
condition where files that were modified twice in the same second will
not be stored correctly.

The --newer-than option does not do the same thing. It's like a
--assume-modified-if-newer-than option.

On Thu, 23 Jan 2014 08:32:14 -0800
Colin Percival cperc...@tarsnap.com wrote:

 On 01/23/14 08:31, Albert Peschar wrote:
  Thanks Nick, your snippet is very helpful.
  
  Point it at anything with a modification time = when the snapshot was
  created.  Obviously anything inside the snapshot will have this property;
  as will a file you create prior to creating the snapshot.
  
  Colin, thanks for your explanation. But I'm definitely misunderstanding
  something here: if I specify a file that was last modified 10 years
  ago, how does that help you?
 
 That will effectively disable the recognize when files haven't changed
 functionality, which will force Tarsnap to re-read files which it might
 otherwise have not bothered to re-read.
 
 -- 
 Colin Percival
 Security Officer Emeritus, FreeBSD | The power to serve
 Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid


Re: Using the --snaptime option

2014-01-24 Thread Colin Percival
On 01/23/14 08:56, Daniel Staal wrote:
 --As of January 23, 2014 8:32:14 AM -0800, Colin Percival is alleged to have 
 said:
 That will effectively disable the recognize when files haven't changed
 functionality, which will force Tarsnap to re-read files which it might
 otherwise have not bothered to re-read.
 
 So, essentially for this purpose it's the same as the `--newer-than` option, 
 and
 could be replaced with any of the --newer options, right?

No, not at all.  As Albert said, it's more of a --assume-modified-if-newer-than
option (although --assume-potentially-modified-if-newer-than would be closer).

 (Of course, we are talking about ZFS snapshots here, which is already have an
 atomic creation and can be browsed like any other filesystem.  I'm not sure if
 that applies to other forms of snapshots.)

Atomic creation doesn't solve the problem of timestamps being too coarse grained
to distinguish between a time just before and a time just after the snapshot was
created.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid